1. Field of the Invention
The present invention relates to a substrate holder, an anodizing apparatus, a semiconductor processing system, and a method of processing or fabricating a substrate and, more particularly, to a substrate holder for holding a substrate to be anodized, an anodizing apparatus including the holder, a semiconductor processing system, and a method of processing or fabricating a substrate.
2. Description of the Related Art
Porous silicon was found by A. Uhlir and D. R. Turner during the course of researching electrolytic polishing of single-crystal silicon biased to a positive potential in an aqueous solution of hydrofluoric acid (to be abbreviated as HF hereinafter).
Thereafter, by focusing attention on a high reactivity of porous silicon, the application of porous silicon to a device isolation step requiring the formation of a thick insulator in the fabrication process of a silicon integrated circuit has been studied, and a complete isolation technique using a porous silicon oxide film (FIPOS: Full Isolation by Porous Oxidized Silicon) and the like have been developed (K. Imai, Solid State Electron 24, 159, 1981).
Also, an applied technique to a direct adhesion technique by which a silicon epitaxial layer grown on a porous silicon substrate is adhered on an amorphous substrate or a single-crystal silicon substrate via an oxide film has been developed recently (Japanese Patent Laid-Open No. 5-21338).
As another application, porous silicon which emits light by itself has attracted attention as so-called photoluminescence and electroluminescence materials (Japanese Patent Laid-Open No. 6-338631).
FIG. 17 is a view showing the arrangement of an apparatus for fabricating porous silicon by anodizing a silicon substrate. In this apparatus, the lower surface of a silicon substrate 1701 is brought into tight contact with a metal electrode 1702, and an anodizing bath 1705 is placed on the silicon substrate 1701 such that the peripheral portion on the upper surface of the silicon substrate 1701 is sealed by, e.g., an O-ring 1704. The bath is filled with an HF solution 1703, and a counterelectrode 1706 is arranged in the bath so as to oppose the silicon substrate 1701. The silicon substrate 1701 is anodized by applying a DC voltage by using the counterelectrode 1706 as a negative electrode and the metal electrode 1702 as a positive electrode.
This method has two major drawbacks. One drawback is that the silicon substrate 1701 is contaminated with the metal because the lower surface of the silicon substrate 1701 is in direct contact with the metal. The other drawback is that a region to be anodized on the surface of the silicon substrate 1701 is only a portion contacting the HF solution, so porous silicon is formed only inside the O-ring 1704.
FIG. 18 is a view showing the arrangement of an anodizing apparatus (Japanese Patent Laid-Open No. 60-94737) developed to solve the above problems. In this anodizing apparatus, HF-resistant Teflon anodizing baths 1802a and 1802b (Teflon is a tradename of Du Pont de Nemours & Co. Inc., U.S.A) are so arranged as to sandwich a silicon substrate 1801. Platinum electrodes 1803a and 1803b are arranged in the anodizing baths 1802a and 1802b, respectively.
The anodizing baths 1802a and 1802b have grooves in side walls contacting the silicon substrate 1801, and O-rings 1804a and 1804b made from fluorine rubber are fitted in these grooves. The anodizing baths 1802a and 1802b and the silicon substrate 1801 are sealed by these O-rings 1804a and 1804b, respectively. The anodizing baths 1802a and 1802b thus sealed are filled with HF solutions 1805a and 1805b, respectively.
In these anodizing baths, as the silicon substrate does not directly contact the metal electrodes; the possibility of the silicon substrate being contaminated by the metal electrodes is low. However, the front and rear surfaces of the silicon substrate to be anodized are sealed by the O-rings. Accordingly, the problem that an unanodized portion remains in the peripheral region of the surfaces of the silicon substrate still remains unsolved. Also, since the silicon substrate to be processed is directly incorporated into and integrated with the anodizing baths, it is impossible to rapidly exchange the silicon substrates.
In consideration of the above problems, an anodizing apparatus which supports a beveling region of a silicon substrate was developed (Japanese Patent Laid-Open No. 5-198556). This anodizing apparatus can prevent contamination from a metal electrode and anodize the entire region of the surface of a silicon substrate. Also, this anodizing apparatus fixes a wafer to be processed in an anodizing bath in two steps in which the wafer is fixed by a holder and the holder is then fixed in the anodizing bath. This greatly improves the operability compared to the conventional apparatus in which a wafer is directly fixed in an anodizing bath to form a part of the anodizing bath.
The anodizing apparatus described in Japanese Patent Laid-Open No. 5-198556 is an extremely practical apparatus which produces almost no metal contamination and can anodize the entire region of the substrate surface.
It is, however, being desired to develop an anodizing apparatus with a higher productivity. For example, when it is necessary to process a large number of types of substrates different in a diameter (e.g., an inch size) or a shape (e.g., an orientation flat or a notch), the anodizing apparatus described in Japanese Patent Laid-Open No. 5-198556 must be equipped with dedicated holders for the individual substrates.
Additionally, in incorporating a substrate into a holder, it is necessary to first match the center of the wafer with the center of a seal surface, match a special-shape portion such as an orientation flat with a corresponding portion of the holder, and then fix the wafer by pushing the seal surface against the periphery of the wafer. Since a considerable press force is required to fix the wafer, screws, for example, are used in the fixation.